1. Field of the Invention
This invention relates to a traction control method for a vehicle, comprising the steps of: judging an excessively slipping state of a plurality of driving wheels by taking into account, as at least one requirement for a judgement of whether or not a driving-wheel speed exceeds a braking-force controlling target wheel speed which is determined based on a vehicle speed by taking into account a predetermined slip rate, and controlling a braking force for the driving wheels in accordance with the result of this judgment.
2. Description of the Related Art
Methods in which an excessive slipping state of the driving wheels is eliminated by applying a braking force to the driving wheels is conventionally already known from Japanese Patent Application Laid-Open No. 109159/91, wherein an excessive slipping state is judged for every driving wheel, and a braking force for each of the driving wheels is independently controlled, in accordance with the judgment made. There is also a known technique in which an excessive slipping state is judged on the basis of an average value between the left and right driving-wheel speeds, and excessive slipping is eliminated by conducting, in, combination, a control of torque applied to the driving wheels in accordance with the result of judgment, and an independent control of braking force for every driving wheel (for example, see Japanese Patent Application Laid-Open No. 203863/87).
In a vehicle having left and right front wheels serving as driving wheels, the following cases can be conceived even when an average speed V.sub.DA between both driving-wheel speeds V.sub.DL and V.sub.DR is substantially equal for the following cases: a case wherein both the driving wheels are in slipping tendencies with both of a left driving-wheel speed V.sub.DL and a right driving-wheel speed V.sub.DR being deviated from a vehicle speed V.sub.R, as shown in FIG. 6; and a Case wherein one of the driving wheels, e.g., the right driving-wheel, is in a slipping tendency, with its speed V.sub.DR being largely deviated from the vehicle speed V.sub.R, while the other driving wheel, e.g., the left driving wheel, is not in a slipping tendency, with its speed V.sub.DL being a value closer to a vehicle speed V.sub.R, as shown in FIG. 7. In a method in which the excessive slipping states of the driving wheels are determined by taking into account, as at least one of judgment requirements, whether or not the driving-wheel speeds V.sub.DL and V.sub.DR exceed the target wheel speed V.sub.RB, based on the judgement the braking forces for the driving wheels are independently controlled, as in the above described prior art, the braking force for the right driving wheel may be increased on the basis that the right driving-wheel speed V.sub.DR largely exceeds the target wheel speed V.sub.RB, even though the average speed V.sub.DA has not been decreased. This results in a possibility of an excessive control of braking force.
In addition, in a traction control method in which an independent control of braking force for every driving wheel and a control of torque applied to the driving wheels in accordance with the result of judgment of the excessively slipping states on the basis of an average value between left and right driving-wheel speeds are combined, it is generally known to carry out the control of torque to the driving wheels by taking into account, as at least one judgment requirement, whether or not the average driving-wheel speed V.sub.DA exceeds the target wheel speed V.sub.RT which is set lower than the target wheel speed V.sub.BR for of the control of braking force, as shown in FIGS. 6 and 7. In this case, the control of braking and the control of torque are conducted in a balanced manner in the condition shown in FIG. 6, whereas in the condition shown in FIG. 7, the change of control of applied torque is reduced because the average driving-wheel speed V.sub.DA is reduced, as shown in FIG. 8, by an increase in braking force for the right driving wheel. In an extreme case, traction control is carried out only by the control of braking force, where the control of torque application is lost, resulting in an excessive control of braking force.